Mark Twain stated that "every one is a
moon, and has a dark side which he never
shows to anybody". Yet it seems that by
gazing upon the dark side of the moon, we
can indeed find  our own reflection - the
sunlit side of the Earth.
Image: iStockphoto

When astronomer Galileo first looked through his telescope and marvelled at the surface of the moon, he launched a lunar fascination that has endured for centuries.

On frosty nights in Mount Macedon Victoria, PhD candidate Sally Langford from the University of Melbourne has, like Galileo before her, marvelled at the surface of the moon, this time discovering that our planet’s only natural satellite, which captures the reflection of the Earth, reveals new information.

The world first study – supervised by Associate Professor Stuart Wyithe from the Astrophysics Group at the University of Melbourne, and colleagues from Princeton University, USA – has shown that the difference in the reflection of light from the Earth’s land masses and oceans can be seen in earthshine: the reflection of the sunlit side of the Earth from the dark side of the moon.

Ms Langford says the brightness of the reflected earthshine varies as the Earth rotates, revealing the difference between the intense mirror-like reflection of the ocean in comparison with the dimmer land.

“In the future, astronomers hope to find planets like the Earth around other stars. However these planets will be too small to allow an image to be made of their surfaces. We can use earthshine, together with our knowledge of the Earth’s surface to reveal the sorts of information that will be available to us from these undiscovered planets,” Ms Langford says.

This is the first study in the world to use the reflection of the Earth to measure the effect of continents and oceans on the apparent brightness of a planet. Other studies have used a colour spectrum and infrared sensors to identify vegetation, or for climate monitoring.

The three-year study involved taking images of the moon over a period of time to measure the Earth’s brightness as it rotated, allowing Ms Langford to detect the difference in signal from land and water.

Observations of the moon were made from Mount Macedon, for around three days each month when the moon was rising or setting. The study was conducted so that in the evening, when the moon was a waxing crescent, the reflected earthshine originated from the Indian Ocean and Africa’s east coast. In the morning, when the moon was a waning crescent – it originated only from the Pacific Ocean.

“When we observe earthshine from the moon in the early evening we see the bright reflection from the Indian Ocean, then as the Earth rotates the continent of Africa blocks this reflection, and the moon becomes darker,” she says.

“If we find Earth-sized planets and watch their brightness as they rotate, we will be able to assess properties like the existence of land and oceans.”